This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to the shimming of the magnetic fields within the MRI system.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and for medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio frequency signals resulting from the precession of the spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are then combined to produce a volumetric image of the nuclear spin density of the body.
A uniform magnetic field is required in the MRI system for image data acquisition. Shim coils are provided in the system which can be selectively energized to improve magnetic field uniformity. Many methods exist for shimming, and the use of a field map to adjust shim currents is a common method. Reference field maps are routinely used to correct for magnetic field inhomogeneity by solving a set of linear equations. While methods for adjustments of linear shim currents for shimming exists, the use of high-order shims has been shown to be of substantial benefit for imaging applications of the brain as well as other body applications. Common approaches for linear shimming include computing optimal linear shim terms for a given metric over the volume of interest and then applying the corresponding currents as constant offsets to the imaging gradients. Similar methods has been investigated for high-order shimming. Calculating the correct field from a reference field map using least squares fit is a typical shimming approach. Variants of this method exist where the current limits of shim coils are taken into account.
Shimming via field mapping methods vary in the way that the field maps are actually obtained. Multiple spin echo methods have been used to obtain the field maps, but faster methods have been used to improve the overall speed of the shimming procedure.
The shim coils in the MRI system are designed as spherical harmonics, which are only guaranteed to be orthogonal over a centered spherical region of interest (ROI). While high-order shimming provides increased homogeneity, shimming in areas that are off-centered or non-spherical can cause numerical instability due to the non-orthogonality of shim fields.
The present invention is directed to an improved method of shimming a magnetic field through use of a reference field map of the MRI system and a volumetric field map of the field of view within an object to be imaged. Shim currents are calculated for the volumetric field map using data from the reference field map and the volumetric field map. Importantly, a regularization method is then applied in recalculating the shim currents for use in energizing the shim coils.
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings.
In accordance with the invention, the shimming of a magnetic field for use in magnetic resonance imaging of a field of view includes obtaining data for a reference field map of the magnetic field, and then obtaining data for a volumetric field map of the field of view within the object. Shim currents are calculated for the volumetric field map using the acquired data. Thereafter, regularization is iteratively applied in re-calculating the shim currents, which are then applied to energize shim coils.
A spiral pulse sequence can be used to acquire the field maps, and a least-squares calculation of shim currents is performed that minimizes the root mean square (RMS) value of the magnetic field inhomogeneity over the volume of interest. A singular value decomposition (SVD) is used to enable regularized methods for solving the least-squares problem. The regularization allows arbitrary regions to be shimmed without the divergence of shim currents.